我需要返回单个字段的所有功能，只返回单个字段，而不是整个数据集。
我的json文件格式如下：
{"type":"FeatureCollection","crs":{"type":"name","properties":{"name":"urn:ogc:def:crs:OGC:1.3:CRS84"}},"features":[{"type":"Feature","properties":{"NAME":"Kingston upon Thames","GSS_CODE":"E09000021","HECTARES":3726.117,"NONLD_AREA":0,"ONS_INNER":"F","SUB_2009":null,"SUB_2006":null,"Area_name":"Kingston upon Thames","Inner_Outer_London":"Outer London","GLA_Population_Estimate_2015":170900,"GLA_Household_Estimate_2015":66870.... ,
{"type":"Feature","properties":{"NAME":"Croydon","GSS_CODE":"E09000008","HECTARES":8649.441,"NONLD_AREA":0,"ONS_INNER":"F","SUB_2009":null,"SUB_2006":null,"Area_name":"Croydon","Inner_Outer_London":"Outer London","GLA_Population_Estimate_2015":380700,"GLA_Household_Estimate_2015":152750,"Inland_Area_Hectares":8650.4,"Population_density_per_hectare_2015":44,"Average_Age_2015":36.8,"Proportion_of_population_aged_0-15_2015":22.1...
编辑：json到json lint（太大了，甚至不能复制粘贴5％）
{
"type": "FeatureCollection",
"crs": {
"type": "name",
"properties": {
"name": "urn:ogc:def:crs:OGC:1.3:CRS84"
}
},
"features": [{
"type": "Feature",
"properties": {
"NAME": "Kingston upon Thames",
"GSS_CODE": "E09000021",
"HECTARES": 3726.117,
"NONLD_AREA": 0,
"ONS_INNER": "F",
"SUB_2009": null,
"SUB_2006": null,
"Area_name": "Kingston upon Thames",
"Inner_Outer_London": "Outer London",
"GLA_Population_Estimate_2015": 170900,
"GLA_Household_Estimate_2015": 66870,
"Inland_Area_Hectares": 3726.1,
"Population_density_per_hectare_2015": 45.9,
"Average_Age_2015": 36.9,
"Proportion_of_population_aged_0-15_2015": 19,
"Proportion_of_population_of_working-age_2015": 68.1,
"Proportion_of_population_aged_65_and_over_2015": 12.9,
"Net_internal_migration_2014": -188,
"Net_international_migration_2014": 2136,
"Net_natural_change_2014": 1178,
"PC_of_resident_population_born_abroad_2014": 29.2,
"Largest_migrant_population_by_country_of_birth_2011": "Sri Lanka",
"PC_of_largest_migrant_population_2011": 2.2,
"Second_largest_migrant_population_by_country_of_birth_2011": "India",
"PC_of_second_largest_migrant_population_2011": 1.8,
"Third_largest_migrant_population_by_country_of_birth_2011": "Korea (South)",
"PC_of_third_largest_migrant_population_2011": 1,
"PC_of_population_from_BAME_groups_2013": 27,
"PC_people_aged_3_whose_main_language_is_not_English_2011_Census": 16.4,
"Overseas_nationals_entering_the_UK_NINo_2014_15": 3157,
"New_migrant_NINo_rates_2014_15": 27.6,
"Largest_migrant_population_arrived_during_2014_15": "Bulgaria",
"Second_largest_migrant_population_arrived_during_2014_15": "Poland",
"Third_largest_migrant_population_arrived_during_2014_15": "Romania",
"Employment_rate_PC_2014": 75,
"Male_employment_rate_2014": 81.6,
"Female_employment_rate_2014": 68.5,
"Unemployment_rate_2014": 5.3,
"Youth_Unemployment_claimant_rate_18-24_Dec-14": 3,
"Proportion_of_16-18_year_olds_who_are_NEET_PC_2014": 3.9,
"Proportion_of_the_working-age_population_who_claim_out-of-work_benefits_PC_May-2014": 7,
"PC_working-age_with_a_disability_2014": 13.8,
"Proportion_of_working_age_people_with_no_qualifications_PC_2014": 3,
"Proportion_of_working_age_with_degree_or_equivalent_and_above_PC_2014": 57.8,
"Gross_Annual_Pay_2014": 36700,
"Gross_Annual_Pay_-_Male_2014": 39359,
"Gross_Annual_Pay_-_Female_2014": 30144,
"Modelled_Household_median_income_estimates_2012_13": 56920,
"PC_adults_that_volunteered_in_past_12_months_2010_11_to_2012_13": 42.7,
"Number_of_jobs_by_workplace_2013": 82300,
"PC_of_employment_that_is_in_public_sector_2013": 17.8,
"Jobs_Density_2013": 1,
"Number_of_active_businesses_2013": 8130,
"Two-year_business_survival_rates_started_in_2011": 79,
"Crime_rates_per_thousand_population_2014_15": 58.5,
"Fires_per_thousand_population_2014": 1.8,
"Ambulance_incidents_per_hundred_population_2014": 11.1,
"Median_House_Price_2014": 385000,
"Average_Band_D_Council_Tax_charge_2015_16": 1674.7,
"New_Homes_net_2013_14": 260,
"Homes_Owned_outright_2014_PC": 27.7,
"Being_bought_with_mortgage_or_loan_2014_PC": 39,
"Rented_from_Local_Authority_or_Housing_Association_2014_PC": 10.7,
"Rented_from_Private_landlord_2014_PC": 22.4,
"PC_of_area_that_is_Greenspace_2005": 36.4,
"Total_carbon_emissions_2013": 752.4,
"Household_Waste_Recycling_Rate_2013_14": 46.3,
"Number_of_cars_2011_Census": 70421,
"Number_of_cars_per_household_2011_Census": 1.1,
"PC_of_adults_who_cycle_at_least_once_per_month_2013_14": 21.1,
"Average_Public_Transport_Accessibility_score_2014": 2.9,
"Achievement_of_5_or_more_A*-_C_grades_at_GCSE_or_equivalent_including_English_and_Maths_2013_14": 68.6,
"Rates_of_Children_Looked_After_2014": 33,
"PC_of_pupils_whose_first_language_is_not_English_2014": 31.2,
"PC_children_living_in_out-of-work_households_2014": 8.9,
"Male_life_expectancy_2011-13": 81.3,
"Female_life_expectancy_2011-13": 84.5,
"Teenage_conception_rate_2013": 15.8,
"Life_satisfaction_score_2011-14_out_of_10": 7.3,
"Worthwhileness_score_2011-14_out_of_10": 7.7,
"Happiness_score_2011-14_out_of_10": 7.4,
"Anxiety_score_2011-14_out_of_10": 3.3,
"Childhood_Obesity_Prevalance_PC_2013_14": 15.4,
"People_aged_17_with_diabetes_PC": 4.9,
"Mortality_rate_from_causes_considered_preventable": 147.8,
"Political_control_in_council": "Cons",
"Proportion_of_seats_won_by_Conservatives_in_2014_election": 58.3,
"Proportion_of_seats_won_by_Labour_in_2014_election": 4.2,
"Proportion_of_seats_won_by_Lib_Dems_in_2014_election": 37.5,
"Turnout_at_2014_local_elections": 43.1
},
"geometry": {
"type": "MultiPolygon",
"coordinates": [
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假设用户想要为所有功能返回HECTARES以及附带的名称，我需要使用哪些代码？
示例：用户输入HECTARES和输出返回类似于：泰晤士河畔金斯顿：3726.117，克罗伊登：8649.441等。
示例2：用户输入GSS_CODE并输出返回类似于：泰晤士河畔金斯顿：＆＃34; E09000021＆＃34;，克罗伊登：＆＃34; E09000008＆＃34;等
到目前为止我的代码看起来像这样：
app.get("/api/boroughs_field", function(req, res){
// read input parameter from request
var field = req.query['field'];
// validate input, check if the parameter is valid or not
if (field === undefined)
{
// return error as a JSON string
var str = JSON.stringify( {"error":"Parameter 'field' is not defined."} );
res.end( str );
}
var fldObj = null;
// make a loop for fields???
if (fldObj == null)
{
// field not found, return an error as a JSON string
var str = JSON.stringify( {"error":"field '"+name+"' doesn't exist. Please check the spelling."} );
res.end( str );
}
// If the program gets here, the borough has been found.
var respJson = {};
respJson["results"] = [ fldObj ];
res.end( JSON.stringify( respJson ) );
});
}
所以是的，我真的不知道如何编写循环来获得所需的结果
答案 0 :(得分：1)
好的，我想你想要下面的东西。我遗漏了很多周围的细节，它们有多种细微差别，但它是你问题的一般答案。
var i, fLength, fieldValue, nameValue;
// HECTARES, GSS_CODE, etc
var requestedFieldName = ???;
// Not quite understanding your return object so you'll have to figure this out
var features = something.features;
// an array of what you were looking for
var res = [];
// Standard loop, instead of checking the length every loop though
// we set it as a constant to start with and check that constant.
for (i=0, fLength = features.length; i<fLength; i++) {
// Javascript objects are a hashmap by their nature,
// normally you can use dot notation myVar.myVal,
// but that only works when myVal is a constant string.
// If it is a variable, as in this case you can use [] instead.
fieldValue = features[i][requestedFieldName];
nameValue = features[i]["NAME"];
res.push({"field":fieldValue", "name":nameValue});
}
答案 1 :(得分：1)
您可以遍历要素列表并使用.hasOwnProperty('propName')搜索对象的属性：
var feature;
var features = [];
for(var i = 0, total = jsonObj.features.length; i < total; i++) {
feature = jsonObj.features[i];
if (feature.hasOwnProperty(field)) {
features.push({ name: feature.name, value: feature[field] });
}
}
我建议您查看lodash以获取更多选项和效果。